1. Field of the Invention
The present invention relates to a continuously variable valve lift system for engines and a controlling method thereof.
2. Description of Related Art
Generally, intake and exhaust valves of an engine have functions to control flow of intake air and exhaust air in a cylinder and maintain airtightness in the cylinder.
That is, both the intake valve and the exhaust valve are closed in a compression stroke and an explosion stroke to maintain the airtightness in the cylinder and the intake valve or the exhaust valve is opened in an intake stroke and the exhaust stroke to take in fuel gas and exhaust combustion gas.
A cam formed in a cam shaft presses ends of the valves through a rocker arm (swing arm) to close and open the valves. The cam shaft rotates by receiving a rotational force of a crankshaft via a timing chain or a timing belt.
Meanwhile, a primary element to determine the airtightness of the valves, amounts of intake gas and exhaust gas, etc. is valve lift, which means a distance in which a valve face is distant from a valve seat.
In general, in the case of the intake valve, as the valve lift is larger, an amount of outside air or fuel gas which is introduced into the cylinder increases in the intake stroke. Contrary to this, in the case of the exhaust valve, as the valve lift is larger, the amount of combustion gas exhausted in the exhaust stroke increases, thereby improving intake and exhaust efficiencies.
Meanwhile, it is a continuously variable valve lift that can continuously vary the valve lift by using a motor and a device having a predetermined configuration.
The continuously variable valve lift (hereinafter, referred to as ‘CVVL’) has been developed in various forms for each automobile manufacturer. Although the CVVL has different names for each automobile manufacturer, each automobile manufacturer has an object to improve an effect (improving engine output and enhancing fuel efficiency) of the CVVL by smoothly switching a high valve lift operation state and a low valve lift operation state into each other and controlling the high and low valve lift operations and a lost motion angle.
However, when a low lift swing angle is larger than a high lift swing angle in the CVVL, a friction loss increases by a return spring during the low lift operation, such that fuel efficiency is deteriorated.
Further, the known CVVL does not have an advancing function of a valve timing or even though the CVVL has the advancing function, the lost motion angle excessively increase, the continuously variable valve lift having the advancing function is difficult to actually implement.
Further, in the case of the CVVL, an assembly clearance is generated at the time of assembling a connection shaft and an oscillating cam link. A difference between a valve profile in an initial design and a valve profile measured in an actual usage is generated due to the assembly clearance.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.